Electrochemical and Chemical Na+/Li+ Ion Exchange in Na-Based Cathode Materials: Na1.56Fe1.22P2O7 and Na3V2(PO4)2F3

In this work, XRD, EDX, Mossbauer and NMR spectroscopy were used to study chemical and electrochemical Na+/Li+ ion exchange in the sodium iron pyrophosphate Na1.56Fe1.22P2O7 with a triclinic symmetry, S. G. P-1, and sodium vanadium fluorophosphate Na3V2(PO4)(2)F-3 with a tetragonal symmetry, S. G. P4(2)/mnm, cathode materials. Electrochemical Na+/Li+ ion exchange was performed in hybrid-ion cells with Li metal anode and LiPF6-based electrolyte, while chemical ion exchange was realized in the solution of LiBr in acetonitrile. A facile electrochemical and chemical Na+/Li+ ion exchange was observed for both cathode materials, resulting in the formation of the mixed Na-Li compositions: similar to Na1.2Li0.36Fe1.22P2O7 and similar to Na2.47Li0.53V2(PO4)(2)F-3, respectively. Partial Na+/Li+ ion exchange did not cause noticeable structural changes to the pristine sodium-based materials. Both materials showed an excellent electrochemical performance in hybrid-ion cells. It was suggested that during cycling, a mixed Na+/Li+ cathode reactions occurred. (C) The Author(s) 2016. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any medium, provided the original work is properly cited. All rights reserved.